Antenna reflector surface contour control



March 8, 1966 J. BANCHE ETAL 3,239,839

ANTENNA REFLECTOR SURFACE CONTOUR CONTROL Filed April 11, 1965 2 Sheets-Sheet l March 8, 1966 J. BANCHE ETAL ANTENNA REFLECTOR SURFACE CONTOUR CONTROL 2 Sheets-Sheet 2 Filed April 11, 1963 4O 3O ELEVATION ANGLE,

United States Patent 3,239,839 ANTENNA REFLECTOR SURFACE CONTOUR CONTROL Joe Banche and David N. Uh-y, Columbus, Ohio, assignors to North American Aviation, Inc. Filed Apr. 11, 1963, Ser. No. 272,424 4 Claims. (Cl..343915) This invention relates to a ground-based precision antenna system having a large-diameter reflector and especially concerns novel means for effecting contour control in the surface portion of the large-diameter reflector used in such system. Antenna systems of the type hereindisclosed are particularly useful in connection with scientific investigations of the ionosphere, emissions from planetary atmospheres, solar corona effects, and various phenomena and problems encountered in navigation, communications, and the like. The invention described and claimed herein has found particular utility in connection with the use of radio equipment having operating capabilities at frequencies up to 10,000 megacycles per second.

Although a ground-based precision antenna system typically employs other major equipment items in addition to a large-diameter primary reflector surface, this invention is essentially concerned with the principal energyreflecting surface contained therein. In order that precision quality and operability be maintained in the antenna system it is necessary to provide a primary reflector construction which obtains an optimum minimization of both surface deflection and total weight throughout a comparatively wide range of surface operating positions. During operation of a typical precision antenna having a large-diameter reflector, the reflector system components are often rotated together through an elevation angle of approximately 90 and through an azimuth angle of approximately 360. Unncessary or improperly distributed weight in the reflector can be the source of excessive surface deflections which result in unsatisfactory system performance. The use of additional material to develop further rigidity in the reflector structure may be the cause of undesirable system tracking inertia and also tends to compound the deflection problem. System performance penalties which cannot be tolerated may thereby be incurred.

The invention detailed and claimed herein has found utility in connection with an antenna system having a primary reflector designed with a diameter of approximately 120 feet and weighing approximately 70,000 pounds. The maximum permissible surface deviation of the primary reflector from a true paraboloid for all operational conditions was not to exceed i0.075 when measured essentially parallel to the reflector axis. The maximum deviation of any point in the reflector with respect to the theoretical or desired reflector surface position was found to be approximately :0.072. This deviation was a net value based upon the most adverse effects of deflection, temperature, and fabrication and placement tolerances.

The net gravity deflection at any point in the reflecting surface of an antenna system large-diameter reflector that rotates in elevation is the resultant of the deflections at that point caused by symmetrical dead-weight loadings and by anti-symmetrical dead-weight loadings, and that resultant deflection varies in magnitude as a function of the reflector elevational angle at particular positions. Accordingly, an antenna system reflector may advantageously be provided with surface deflection compensation means responsive to reflector elevational angle changes to thereby effect better reflector performance and improved reflector surface contour control throughout the reflector elevational range.

A primary object of this invention is to provide an an- "ice tenna system primary reflector with an improved surface contour control capability.

Another important object of our invention is to provide an antenna system primary reflector with surface deflection compensation means that responds as a function of the antenna reflector elevational angle.

A still further object of this invention is to provide an antenna reflector with surface deflection compensation means that may be readily constructed into embodiments that utilize fixed-value gravitational forces only in achieving a reflector surface contour control as the antenna reflector is moved throughout its elevational range.

Other objects and advantages of this invention will become apparent during a consideration of the description and drawings of this application.

In the drawings:

FIG. 1 is a rear elevational view of an antenna system which incorporates a preferred embodiment of our invention;

FIG. 2 is a sectional view taken generally along line 2-2 of FIG. 1;

FIG. 3 is a perspective view of a portion of the deflection compensation means incorporated in the primary reflector of the antenna system of FIGS. 1 and 2;

FIG. 4 is a detail elevational view of another portion of said deflection compensation means;

FIG. 5 provides a graphical presentation of antenna reflector actual surface deflections and desired compensation deflections as a function of antenna reflector elevational angle; and

FIG. 6 is a schematic illustration of an alternate form of the deflection compensation means of our invention.

The major components of a Cassegranian-type antenna system 10 illustrated completely in FIG. 1 and partially in FIG. 2 are: base 11, elevation-azimuth mount 12, primary reflector 13, and ballast or counter-weight means 14. Primary reflector 13 is essentially comprised of primary backup structure 15 and contoured reflecting surface 16. Means for effecting antenna reflector deflection compensation and improved surface contour control as a function of the primary reflector elevational angle is referenced generally as 17 in such figures. The form of deflection compensation means illustrated in FIGS. 1 through 4 of the drawings makes use of fixed-value gravitational forces for achieving the desired end results. Mechanically powered means in the form of hydraulicallyloaded actuators and command elements which respond to the antenna system elevational angle are shown as an alternate embodiment of this invention in FIG. 6. Also, this invention has application to large-diameter reflectors incorporated in antenna systems of other than the Cassegranian-type.

In the drawings, the various reference lines for the antenna system are designated by the numeral 18. In all positions of the antenna, reference line 18 corresponds to the radio-frequency axis of the system. Referring to FIG. 2, the antenna is considered in its zenith position when the radio-frequency axis corresponds to line 18a and is in a true vertical condition. 18b designates the position of the antenna radio-frequency axis when the system is in a true horizontal or face-forward condition. designates an intermediate position which is 0 from the position of 1812. The angle 0 is measured from 18b only for convenience. It also relates to the FIG. 5 graphical analysis of experienced antenna reflector load deflections and effected compensation deflections.

Portions of a quadripod structure 19 that supports the Cassegranian-type antenna system secondary reflector are shown in FIG. 2. Both the reflector backup structure and reflector surface portions (15 and 16) of the system are generally symmetrical about the radio-frequency axis 18. In the illustrated antenna system, five concentric truss-type ring beams through 24 are connected to each other by inter-ring tension members (not shown). The secondary or auxiliary backup structure 25 carries reflector portions 15 and 16 and is basically provided to etficiently restrict the distortion of those reflector portions as normally caused by anti-symmetrical gravitational loadings. Additional details regarding the interring tension members and the extent of secondary backup structure 25 are provided in US. Patent No. 3,105,969, issued October 1, 1963.

FIGS. 3 and 4 provide details regarding the deflection compensation means 17 incorporated in the antenna primary reflector and illustrated generally in FIGS. 1 and 2. Such deflection compensation means includes a support bracket which is rigidly coupled to ballast 14 and variably-loaded lever members 31 and 32 which are carried by and rotatable relative to bracket 30 through connectors 33 and 34. Such connectors serve as the rotational axis for the lever members. Each lever member carries a fixed weight assembly 35 or 36 to establish the basic magnitude of desired deflection compensation forces. Suitable means are also provided, but not referenced, for conveniently adjusting or changing the position of the weight assemblies relative to rotational axes 33 and 34. However, changing the size or position of the weight assemblies does not change the basic quality or function of the deflection compensation means.

Each lever member carries one or more connector devices 37 for coupling the gravity compensation forces developed by the lever member to other portions of the deflection compensation means and to antenna reflector surface 16. Other coupling means consists of cables 38 and 39 and bellcranks 40. Bellcrank 40 is rotatably connected to the reflector secondary backup structure 25 by connector means 41. Cables 38 may be connected to bellcranks 40 using conventional clevis-type fastener devices. Cables 39 may be coupled to bellcranks 40 and to primary backup structure 15 by conventional clevistype fastener devices also. It should be noted that the desired deflection compensation forces which originate with the disclosed lever and weight members are transmitted into backup structure 15 and connected surface 16 so as to be applied in a direction parallel to radiofrequency axis 18. To achieve this object it is only necessary that cable components 39 be oriented parallel to that radio axis. Undesired components of the reaction forces present at connector 41 are preferably transmitted into selected components of secondary backup structure 25 where they will have a minimum effect on antenna surface deflection.

The gravitational forces applied to weight members 35 and 36 and components attached thereto cause rotational moments in lever members 31 and 32 about axes 33 and 34 and at all reflector positions other than true zenith. The reaction moments thereto are transmitted into the various cables 38. If desired, pre-load forces may be induced into compensation means 17 by tensioning cables 38 when reflector 13 is in its zenith position. It should be noted that the locations for attaching cable members 39 relative to backup structure 15 and surface 16 and the desired weight values and moment arms for deflection compensation means 17 may be selected and determined by conventional mathematical analysis of the reflector structure. As will be developed in the discussion of FIG. 5, the positioning and location of the various components does not alter the functional characteristics of deflection compensation means 17.

In FIG. 5, we have graphically illustrated the relation of various reflector deflection components and desired compensation forces. The abscissa values given on the graph correspond to the angle 0 of FIG. 2. No units are specified for the deflection ordinates. Curve plots antenna reflector surface deflections caused by antisymmetrical gravitational loadings as the reflector portion of the antenna system is rotated through its elevational range. Such deflections are equal to the deflections associated with the true horizontal position (radio-frequency axis 18b) as a function of l-sine 0. Curve 51 plots antenna reflector surface deflections caused by symmetrical gravitational loadings as the system is rotated through its elevational range. Such deflections are equal to the deflections associated with a true vertical or zenith position (radio-frequency axis 18a) as a function of cos 9. Curve 52 represents the dead-weight deflection sum of curves 50 and 51. Curve 53 is the negative of curve 52, plots the desired compensation deflection, and is representative of the changing deflection compensation force that should be introduced into the antenna reflector primary backup structure and surface as the system is rotated to the various indicated elevation angles. The desired net deflection compensation force as noted approaches being a linear function of the system elevational angle. In practice, each point on the reflector surface behaves according to such relationships though generally having different deflection amplitudes. It generally suffices to match the deflection compensation system of our invention to the more significant points of surface deviation although the means may be extended to include surface points of secondary interest.

We have provided an extremely reliable and comparatively simple means for improving the reflecting surface contour control capability of a large-diameter antenna reflector as the incorporating antenna system is rotated in elevation throughout a substantial range. Gravitational forces alone may be utilized in developing satisfactory compensation. In the zenith position lever members 31 and 32, for instance, are often supported entirely by the antenna ballast 14 and no compensation forces are developed for transmission through cables 38 and 39 and bellcranks 40 into the antenna primary backup structure 15 or reflector surface 16. As the antenna is rotated to its full face-forward position or to intermediate positions, the desired compensation force for particular points is developed in lever means 31, 32. The weight value for each lever member remains fixed throughout antenna operation; however, the effective value of the moment arm applying the compensation force developed from gravitational forces acting on weight assemblies 35, 36 varies as a function of the reflector elevational angle and reaches a maximum value when 0 equals 0. The so-developed and so-applied deflection compensation forces will follow curve 54 according to a l-sine 0 function, are non-linear relative to elevational displacement of the reflector, and are sufficiently close to the curve 53 function for all antenna reflector positions so as to develop the desired reflecting surface contour control capability.

An alternate form of deflection compensation means 17 utilizing the features of our invention is illustrated schematically in FIG. 6. The arrangement of this illustration employs a mechanically-powered system to achieve the results associated with the deflection compensation means 17 of FIGS. 1 through 4. Referring to FIG. 6, an actuator 60 powered by pressurized hydraulic fluid is installed in series in each cable 38 to develop tension forces intermediate fixed structure such as ballast 14 and the movable bellcrank 40. A potentiometer-type hydraulic valve 61 is employed to control the fluid pressure to actuator means 61). The command signal for control of valve 61 originates with the cam member designated 62. A conventional source of pressurized hydraulic fluid is designated generally as 63 and is basically comprised of a properly connected pump P and reservoir R. Actuator 60 has a rod or jack stem 64 which is movable by a connected piston element or the like in actuator 60 to change the stress induced in cable 38. A movable sensor element 65 contacts the surface 66 of cam 62 and is connected to the valve spool of potentiometer-type hydraulic valve 61. Cam 62 is fixed to support structure 12 so that during antenna system opation the antenna reflector and other components of the FIG. 6 form of deflection means 17 rotate relative thereto. The indicated center of cam 62 corresponds to the elevational axis of rotation for the antenna system. The surface 66 of cam 62 is generally made to depart from a circular circumference condition throughout an elevational range of approximately 90 to give a true linear function of the antenna elevational position or to give a function corresponding identically to curve 53 or curve 54 of FIG. 5. Thus, surface 66, in combination with the other elements shown in FIG. 6, provides the same improved reflecting surface contour control capability characteristically associated with previously-described lever members 31 and 32 and the weight assemblies carried thereby. Hydraulic fluid pressurized as a selected function of the antenna reflector elevational angle through valve 61 and cam 62 cooperation is ported to actuator 60 by means of the hydraulic fluid line designated 67. The supply and return hydraulic fluid lines between valve 61 and the pump reservoir combination designated 63 are comprised of hydraulic lines 68 and 69. It may be required that the latter hydraulic lines he provided with a flexible construction. This is for the reason that valve 61 is mounted rigidly relative to reflector backup structure 25 whereas the hydraulic system power components 63 may be located in a fixed posi tion in the manner of cam 62 on mount 12. In either event, the components of the FIG. 6 arrangement may be readily provided to function with the same end results achieved by the form of our invention illustrated in FIGS. 1 through 4.

It is to be understood that the forms of the invention herewith shown and described are to be taken as preferred embodiments of the same, but that various changes in the shape, size, number, and arrangement of parts may be resorted to without departing from the spirit of the invention or the scope of the subjoined claims.

We claim:

1. In an antenna system:

(a) .a contoured reflecting surface having a radiofrequency axis and having a surface point which is deflected from an established reference point in a direction parallel to said radio-frequency axis when the reflecting surface is rotated in elevation from a position with said radio-frequency axis in a vertical condition,

(b) primary backup structure fixedly connected to and providing support for said reflecting surface,

(c) secondary backup structure fixedly connected to and providing support for said primary backup structure,

(d) separate means cooperating with said secondary backup structure and originating a deflection compensation force which varies in magnitude from a minimum value when said reflecting surface is positioned with said radio-frequency axis in a vertical condition to a maximum value as a function of the angular displacement of said radio-frequency axis from said vertical condition, and (e) cable means connected to said separate means and to said primary backup structure and transmit- 5 ting said deflection compensation force from said separate means to said primary backup structure in the region of said surface point, said secondary backup structure being relatively stabilized against deflection in comparison to said primary backup structure, and said cable means applying said compensation force to said primary backup structure along a direction which is parallel to said radio-frequency axis.

2. The antenna system defined by claim 1, wherein said separate means consists of a lever member rotatably supported by said secondary backup structure at a rotational axis, and weight means connected to and carried by said lever member a distance from said lever member rotational axis, said lever member and weight means developing a moment about said lever member rotational axis that originates said deflection compensation force and that varies as a l-sine 0 function as said reflecting surface, primary backup structure, and secondary backup structure are rotated together from said radio-frequency axis vertical condition, 0 being the complement of the angular displacement of said radio-frequency axis from said vertical condition.

3. The antenna system defined by claim 1, wherein said separate means consists of a movable valve member 30 linearly controlling the force loading of a hydraulic actuator connected in force-originating relation to said cable means, and a cam surface engaging and controlling the movement of said valve member, said cam surface varying in distance from the rotational axis of said secondary backup structure as a linear function of the angular displacement of said radio-frequency axis from said vertical condition.

4. The antenna system defined by claim 1, wherein said secondary backup structure includes fixedly connected counter-weight means at a side opposite to said primary backup structure and said reflecting surface, said separate means developing a reaction force opposite to said deflection compensation force at said counter-weight 4r means.

References Cited by the Examiner UNITED STATES PATENTS 2,985,881 5/1961 Holland et al. 343-912 3,153,789 10/1964 Ashton 343-765 FOREIGN PATENTS 1,039,252 9/1958 Germany.

ELI LIEBERMAN, Acting Primary Examiner.

HERMAN KARL SAALBACH, Examiner.

P. L. GENSLER, Assistant Examiner. 

1. IN AN ANTENNA SYSTEM: (A) A CONTOURED REFLECTING SURFACE HAVING A RADIOFREQUENCY AXIS AND HAVING A SURFACE POINT WHICH IS DEFLECTING FROM A ESTABLISHED REFERENCE POINT IN A DIRECTION PARALLEL TO SAID RADIO-FREQUENCY AXIS WHEN THE REFLECTING SURFACE IS ROTATED IN ELEVATION FROM A POSITION WHEN SAID RADIO-FREQUENCY AXIS IN A VERTICAL CONDITION, (B) PRIMARY BACKUP STRUCTURE FIXEDLY CONNECTED TO AND PROVIDING SUPPORT FOR SAID REFLECTING SURFACE, (C) SECONDARY BACKUP STRUCTURE FIXEDLY CONNECTED TO AND PROVIDING SUPPORT FOR SAID PRIMARY BACKUP STRUCTURE, (D) SEPARATE MEANS COOPERATING WITH SAID SECONDARY BACKUP STRUCTURE AND ORIGINATING A DEFLECTION COMPENSATION FORCE WHICH VARIES IN MAGNITUDE FROM A MINIMUM VALUE WHEN SAID REFLECTING SURFACE IS POSITIONED WITH SAID RADIO-FREQUENCY AXIS IN A VERTICAL CONDITION TO A MAXIMUM VALUE AS A FUNCTION OF THE ANGULAR DISPLACEMENT OF SAID RADIO-FREQUENCY AXIS FROM SAID VERTICAL CONDITION, AND (E) CABLE MEANS CONNECTED TO SAID SEPARATE MEANS AND TO SAID PRIMARY BACKUP STRUCTURE AND TRANSMITTING SAID DEFLECTION COMPENSATION FORCE FROM SAID SEPARATE MEANS TO SAID PRIMARY BACKUP STRUCTURE IN THE REGION OF SAID SURFACE POINT, SAID SECONDARY BACKUP STRUCTURE BEING RELATIVELY STABILIZED AGAINST DEFLECTION IN COMPARISON TO SAID PRIMARY BACKUP STRUCTURE, AND SAID CABLE MEANS APPLYING SAID COMPENSATION FORCE TO SAID PRIMARY BACKUP STRUCTURE ALONG A DIRECTION WHICH IS PARALLEL TO SAID RADIO-FREQUENCY AXIS. 